In the past, there has been known an electrodischarge machining method which uses an electrodischarge machine to form a hole of a diffuser shape (for example, see PLT 1). In the machining method which is described in this PLT 1, an electrode holder is used to hold a pipe shaped pipe electrode, the pipe electrode is made to move relative to the workpiece in the XY plane while making it advance in a plate thickness direction (Z-direction), and a through hole which has a diffuser shaped taper (gradient) part and straight part which is connected with the taper part is formed in the workpiece. For the pipe electrode, one which has an electrode diameter of the same extent as the minimum width of the straight part is used.
When forming a cross-sectional rectangular shaped through hole using the machining method which is described in PLT 1, the corner parts of the inside wall surfaces of the taper part become arc shapes with radii of curvature corresponding to the electrode diameter of the pipe electrode, so the shape of the taper part is restricted by the electrode diameter. For this reason, it is difficult to form, for example, arc parts of radii of curvature smaller than the electrode diameter of the pipe electrode at the inside wall surfaces of the taper part or otherwise form the taper part into various shapes.